linux-kernel - Re: [RFC] exponential update_wall

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Date:	Wed, 27 Sep 2006 16:13:12 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Roman Zippel <zippel@...ux-m68k.org>
Cc:	lkml <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...l.org>
Subject: Re: [RFC] exponential update_wall_time

On Thu, 2006-09-28 at 01:04 +0200, Roman Zippel wrote:
> On Wed, 27 Sep 2006, john stultz wrote:
> 
> > Accumulate time in update_wall_time exponentially. 
> > This avoids long running loops seen with the dynticks patch
> > as well as the problematic hang" seen on systems with broken 
> > clocksources.
> 
> This is the wrong approach, second_overflow() should be called every HZ
> increment steps and your patch breaks this.

First, forgive me, since I've got a bit of a head cold, so I'm even
slower then usual. I just don't see how this patch changes the behavior.
Every second we will call second_overflow. But in the case where we
skipped 100 ticks, we don't loop 100 times. Could you explain this a bit
more?


> There are other approaches oo accommodate dyntick. 
> 1. You could make HZ in ntp_update_frequency() dynamic and thus reduce the 
> frequency with which update_wall_time() needs to be called (Note that 
> other clock variables like cycle_interval have to be adjusted as well). 

I'm not sure how this is functionally different from what this patch
does.


> 2. If dynticks stops the timer interrupt for a long time, it could 
> precalculate a few things, e.g. it could complete the second and then 
> advance the time in full seconds.

Not following this one at all.

Again, sorry for being so thick.
-john


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